A PAA comprises N antennas disposed along the three spatial dimensions. The antennas which form the PAA are known as elementary antennas. A PAA allows the optimization and the beamforming of the radiated electrical field through the adjustment of the amplitude and phase of the radio frequency (RF) signals sent to each elementary antenna. This is an exclusive feature for the PAA's, since in an individual antenna the optimization of the radiated electrical field depends on the antenna's design. In addition, the beamforming of the radiated electrical field depends on the spatial orientation of the antenna.
As previously said, the beamforming of the electrical field radiated by the PAA can be controlled through the phase of the RF signals applied to each elementary antenna. More precisely, the PAA can be fed by a single RF signal, which is split to all elementary antennas including a tunable phase shift. Although tunable, the phase shift is constant. This means the induced phase shift value is correct only for a specific RF frequency. In a PAA, this implies that the emitted frequency should be constant. Otherwise, different RF frequencies get different phase shifts, and so the beamforming of the electric field becomes dependent on the emitted frequency. In the case of data transmission over one RF carrier frequency, this implies that the data signal bandwidth should be as low as possible. Since there are many applications with high bandwidth (e.g., RADAR signals, Gb/s wireless networks, radio astronomy, etc.), the beamforming of the radiated electric field cannot be efficiently performed with such a technique.
This problem can be solved using phase shifts that depend on RF frequency. In practice, a phase shift depending on frequency consists in a time delay line. Therefore, instead of a phase shift, each elementary antenna should include a tunable time delay. The electrical implementation of a tunable time delay is particularly challenging at high frequencies, since increasing time delays implies increasing the length of the delay line, which in turn leads to greater insertion losses and a reduction in bandwidth. Such disadvantages are overcome using photonic implementations of tunable delay lines. The advantages of photonic systems consist on low losses, broad bandwidth, lighter weights, smaller dimensions and immunity to electromagnetic interference. Generally, a PAA with tunable photonic delay lines is characterized in that it has an electro-optical modulator, which converts the RF signal to the optical domain, followed by an optical processing system which delays and distributes the modulated optical signal according to what the needs of the different antennas are. The optical signals are converted to the electrical domain using photodetectors.
The patents summarized below describe different implementations of photonic TODL's, which can be applied to PAA.
U.S. Pat. No. 5,428,218 discloses a photonic TODL based on spatial multiplexing. In such free-space implementation, the optical signal is directed into a given optical fiber through the adjustment of mirrors. As different optical fibers have different lengths, one can get a discrete tuning of the time delay added to the optical signal. This implementation also includes the possibility of having a multi-beam system, e.g., the system can be simultaneously used by more than one optical signal.
U.S. Pat. No. 5,978,125 discloses a photonic TODL based on polarization multiplexing. In a birefringent medium, an optical signal with a specific state-of-polarization (SOP) has a higher time delay than the orthogonal SOP. By selecting one of the two SOP's, the added time delay can then be controlled. This method is described in a cascaded configuration, in which serial birefringent media are intercalated by polarization controllers (PC's). As a result, a discrete delay tuning is obtained.
U.S. Pat. No. 5,461,687 discloses a photonic TODL based on dispersive means. By tuning the wavelength of the input optical signal the path by which the signal propagates changes, resulting in a tunable time delay. In this patent, the dispersive mean is implemented in a free space using a diffraction grating. Another possible option is to use fiber Bragg gratings (FBG's) located in different points of an optical fiber.
U.S. Pat. No. 5,751,466 discloses a photonic TODL which uses the frequency response of a photonic bandgap device. An example of such a device is a FBG. This device consists of a dielectric structure in which the refractive index varies longitudinally. The control of the refractive index's variation along the structure results in changing the frequency response of the device, therefore affecting the time delay added to the photonic signal.
U.S. Pat. No. 7,558,450 B2 discloses a photonic TODL composed by three resonant elements coupled to a waveguide. This implementation is limited to single sideband (SSB) optical signals. The symmetric displacement in the frequency of two resonant elements enables the adjustment of time delay induced to the RF carrier. The third resonant element adjusts the phase of the optical carrier (OC) in order to avoid involuntary phase shifting of RF signal. Of greater importance than the tuning method, is the fact that this patent is clearly appropriate to RF signals without any spectral content between the optical and the RF carriers. As a result, the TODL's frequency response is irrelevant at frequencies located between the RF carrier and the OC.